In order to improve a power generating efficiency in a power generating plant, a steam turbine installed in the power generating plant is also required to improve efficiency. FIG. 17 is a view illustrating a part of meridian cross section of a conventional steam turbine 300.
FIG. 17 illustrates one turbine stage 310. This turbine stage 310 is configured by a stationary blade cascade 320 and a rotor blade cascade 330 positioned at an immediately downstream side of the stationary blade cascade 320. The stationary blade cascade 320 includes a plurality of stationary blades 323 supported with a predetermined interval therebetween in a circumferential direction between a diaphragm inner ring 321 and a diaphragm outer ring 322. The rotor blade cascade 330 includes a plurality of rotor blades 331 implanted, in a rotor disk 341 provided to a turbine rotor 340, with a predetermined interval therebetween in the circumferential direction.
In each turbine stage, a pressure P1 of steam 350 at an inlet of the stationary blades 323 is reduced since the steam passes through the stationary blades 323, and the pressure P1 becomes a pressure P2 at an outlet of the stationary blades 323. At this time, the steam 350 expands and increases its volume, and at the same time, a steam outflow direction is changed to a rotational direction of the turbine rotor 340, resulting in that the steam 350 has a velocity energy in the circumferential direction.
By a reaction force obtained when the direction of the steam 350 is changed to a counter-rotational direction by the rotor blades 331, and also by a reaction force obtained when the pressure is reduced to a pressure P3 so that the steam further expands and increases its outflow velocity, the velocity energy in the circumferential direction is converted into a rotational torque.
Here, it is structurally essential to provide a predetermined gap between a static part such as the diaphragm inner ring 321 and a rotating part such as the turbine rotor 340. For this reason, a leakage steam 351 whose flow is divided from the steam 350 passes through a gap 360 between the diaphragm inner ring 321 and the turbine rotor 340, as illustrated in FIG. 17. Concretely, the leakage steam 351 passes through the gap 360 between a sealing part 324 provided on an inside of the diaphragm inner ring 321 and the turbine rotor 340.
The leakage steam 351 does not flow through the stationary blade cascade 320, so that the leakage steam 351 on which the predetermined change in the direction is not performed is directly jetted from a portion between the diaphragm inner ring 321 and the rotor blades 331 toward a main flow to interfere with the main flow, which results in generating a loss.
A difference between the pressure P2 and the pressure P3 in front of and at the rear of the rotor blades 331 becomes a force that pushes the rotor disk 341 including the rotor blades 331 toward a turbine rotor axial direction. This force has a substantial magnitude in the entire steam turbine configured by multi-turbine stages. The force is normally cancelled by a thrust bearing with large diameter.
Among conventional steam turbines, one that includes a configuration different from that of the above-described steam turbine 300 has also been considered. FIG. 18 is a view illustrating a part of meridian cross section of a conventional steam turbine 301. Note that a component part same as that of the steam turbine 300 illustrated in FIG. 17 is denoted by the same reference numeral, and an overlapping explanation thereof will be omitted.
As illustrated in FIG. 18, there is formed, on the rotor disk 341 in the steam turbine 301, a steam passage 342 through which the leakage steam 351 is led from an upstream side to a downstream side of the rotor disk 341. With this configuration, a flow rate of the leakage steam 351 jetted from a portion between the diaphragm inner ring 321 and the rotor blades 331 toward a main flow is reduced. For this reason, a loss generated when the steam 351 is jetted toward the main flow is reduced. Further, a differential pressure (P2−P3) in front of and at the rear of the rotor blades 331 becomes small. For this reason, a force applied to the thrust bearing also becomes small, so that it is possible to reduce a diameter of the thrust bearing.
Here, FIG. 19 and FIG. 20 are views schematically illustrating secondary flow vortices generated on a root side of the rotor blades 331 in the conventional steam turbine 300. Note that FIG. 19 is a perspective view in which the vortex is seen from a trailing edge side of the rotor blades 331, and FIG. 20 is a plan view in which the vortices are seen from a tip side of the rotor blades 331.
Generally, a pressure between the rotor blades 331 becomes high on a pressure side 332 (pressure surface side), and it becomes low on a suction side 333 (suction surface side). Further, a driving force 370 of secondary flow vortex acts from a position with high pressure to a position with low pressure. Normally, a centrifugal force obtained when a steam flows between the rotor blades 331 while a direction thereof is changed acts so as to counter the driving force 370. Meanwhile, in the vicinity of an annular wall surface 334 on the root side, a flow velocity of the steam is significantly lowered due to a friction between the steam and the wall surface 334. Accordingly, the centrifugal force is lowered and cannot counter the driving force 370, resulting in that the secondary flow vortex is generated. The secondary flow vortex is classified into a horseshoe vortex 371 generated at a leading edge portion of the rotor blade 331 and developed along the suction side 333, and a passage vortex 372 developed while being drawn from the pressure side 332 toward the suction side 333 by the driving force 370. Both of the vortices sterically cross each other at a rear flow part of the suction side 333, and generate a large loss while curling up in a blade height direction.
As described above, when the steam passage 342 is not formed on the rotor disk 341 in the conventional steam turbine, there is generated the loss due to the interference of the steam 351 leaked between the diaphragm inner ring 321 and the turbine rotor 340 with the main flow. Further, the thrust bearing with large diameter is required to support the force generated due to the difference between the pressure P2 and the pressure P3 in front of and at the rear of the rotor blades 331, which increases manufacturing cost.
On the other hand, when the steam passage 342 is formed on the rotor disk 341 in the conventional steam turbine, it is possible to suppress the loss due to the interference described above, and to downsize the thrust bearing. However, the amount of steam that flows into the rotor blades is reduced, so that the blade height of each rotor blade set based on the flow rate of the steam becomes low. For this reason, the secondary flow vortex occupies a large area in the blade height direction, resulting in that the influence of loss caused by the secondary flow vortex becomes large.